Magnetic materials have been commonly and widely used in or applied to magnetic tapes and disks, recorders, magnetic switches, and seals, etc. Recent years, applying magnetic materials to new fields has been developing, the new fields including the medicine preparation, the purification of protein and DNA in biomedical field, and the management and treatment of environmental waste. For example, by using magnetic materials to result magnetically responsive spheres disclosed by U.S. Pat. No. 4,687,748, the application of magnetic materials in cell separation, affinity purification or immunochemical assays is possible. The separation of a mixture to have the desired component or product is, therefore able to be reached by using a properly magnetic material, together with its related technical skills.
The magnetic materials, as mentioned above, are magnetic liquids, also so-called “ferrofluids.” In view of a target subject treated by such a magnetically separation technique, techniques with under external magnetic field, have been developed as follows: (1) when the sample having magnetic characteristic, separating the magnetically desired component from the sample, and (2) when the sample not having magnetic characteristic, prior to the process of separation, performing the combination of the sample with a magnetic material. The magnetic material in (2) will be a key in the combination process.
The process for preparing the magnetic material varies with different field and needs, and includes techniques of (1) mechanical grinding, for example, U.S. Pat. No. 4,604,222 disclosing the method of ball milling and grinding for preparing the ferrofluid composition by mixing ferromagnetic particles, a cationic surfactant, and organic liquid carriers such as glycol or ester, and then grinding the obtained mixture, the ferrofluid composition obtained being used to improve electrical conductivity in sealing computer disc drives and in sputtering apparatus in semiconductive industry; (2) oxidation, for example, U.S. Pat. No. 6,140,001 disclosing iron oxide magnetic particles prepared by oxidation of oxygen and ferrous (II) hydroxide, including by mixing a solution of a soluble phosphate compound, such as sodium orthophosphate, a solution of ferrous ion, with a solution of hydroxide of an alkali metal or of an alkaline earth metal; and (3) chemical co-precipitation, for example, U.S. Pat. No. 6,743,371 disclosing a magneto sensitive fluid composition exhibiting electrical switching as well as magnetorheological characteristics in the presence of external magnetic field, the composition prepared by mixing nickel-zinc ferrite or manganese-zinc ferrite and any conductive metallic or non metallic powder such as silver, graphite powder.
However, the disadvantages also exit in above processes, the timing for operation mechanical grinding taking around 2-6 weeks, chemical co-precipitation creating chemical waste, comprising un-reacted metal salt solutions and uncoated particles in aqueous and nonaqueous media needed to be disposed of in proper compliance with environmental regulations. The waste removal adds to the cost of manufacturing the ferrofluids.
Regarding the process for preparing magnetic materials, specifically, a ferrofluid, surface treatment of magnetic particles is necessary, so as to effectively avoid aggregation of particles due to attraction of magnetic characteristics thereof, and improve the dispersion in the liquid carrier. The way of surface treatment, for example, is different from water- or oil-based ferrofluids.
In terms of preparation of oil-based ferrofluids, U.S. Pat. No. 5,124,060 disclosing a magnetic fluid prepared by adding the low boiling organic solvent and the dispersant having oleophilic groups to fine ferromagnetic particles to obtain an intermediate medium, separating the fine particles of poor dispersibility from the intermediate medium and, then adding the less volatile organic solvent and heating the resulting material to evaporate the low boiling organic solvent. The magnetic fluid composition can be used for sealing devices in computer hard disk driver or vacuum apparatus. Another example disclosed by U.S. Pat. No. 6,068,785, the oil-based ferrofluid is prepared by grinding a slurry formed of particles of a non-magnetic oxide of iron α-Fe2O3, an oil carrier liquid, such as Ampro Type II oil, and a surfactant, such as polyolefin anhydride. Again, their non-economical impact is the disadvantage.
Furthermore, the application of conventionally oil-based ferrofluid, as mentioned previously, is primarily limited to magnetic memory device, sealing devices in computer hard disk driver or vacuum apparatus, but not to organism. While, in addition to the disadvantages of the process for preparing ferrofluid, as mentioned previously, α-Fe2O3 used in the process, as mentioned above, for preparing ferrofluid is not a magnetic conductive material and is hard to perform magnetic transfer. Likewise, the unsuitable for many applications of water-based ferrofluids is left the room to improve the preparation and application of ferrofluid.
Accordingly, there is a need for a ferrofluid suitable for being used in an organism, and a process thereof is economical and efficient.